Endoscopy procedures allow clinicians to visualize internal organs in a much less invasive manner than exploratory surgeries. The use of optical fibers reduces the size and increases the flexibility of endoscopes, thus potentially allowing safer, faster and cheaper office-based procedures. Advances in endoscopy, such as imaging with coherent fiber bundles, have also paved the way towards endomicroscopy, which is an imaging technique combining the flexibility of endoscopy with the advantages of confocal microscopy, namely micrometric resolution and optical sectioning.
Reflectance single fiber endoscopy is however subject to speckle noise as the use of lasers and single-mode (SM) fibers results in imaging that is both temporally and spatially coherent. The use of a double-clad fiber (DCF) dramatically reduces speckle contrast while improving signal collection. In order to preserve lateral resolution, the SM core of the DCF is used for illumination, while the multi-mode (MM) inner cladding is used for collection of partially incoherent light reflected from the sample. Coupling light in and out of a DCF is typically performed using a free space beam splitter setup resulting in a greater than 6 dB loss of the weak SM signal and a greater than 3 dB loss in the MM signal, making the system quite vulnerable to misalignment due to mechanical motions.
Therefore, despite the advances made, coupling remains an issue for the use of DCF in medical imaging techniques.